Affiliations

Goal

Headquarters

Fate

Nightmares are sinister horse-like beings made out of blacksand -- originally Sandman's dreamsand corrupted and manipulated by Pitch. Onyx is Pitch's pet, formerly Cupcake's own dream of a unicorn.

In the Rise of the Guardians App, Dash n Drop, there's also another nightmare creature that resembles a large bat.

In the Rise of the Guardians: The Video Game, there are many more Nightmare creatures based on scary animals. Wolves, Bats, Rats, Killer Lobsters, Giant Squids, Wasps, Hideous Monkeys, Nightmare Orchids and more.

Trivia

Onyx's fate was that Cupcake restored her back to a Dreamsand Unicorn.

Onyx's name was never mentioned in the film, except in the film's screenplay.